1. Field of the Invention
The invention relates to the field of audio signal processing.
2. Prior Art
Two of the most common maladies afflicting audio signals are excessive noise and insufficient high-frequency energy. There have been many efforts to eliminate noise by applying the signal to a lowpass filter whose bandwidth is controlled by the high-frequency content of the program material. When the program contains large amounts of high frequency energy, the filter's bandwidth opens because this high frequency energy psychoacoustically masks the noise. When the program contains little high frequency energy, the bandwidth of the filter decreases to reduce audible noise. U.S. Pat. Nos. 4,609,878 and 4,696,044 contain a good list of references on this subject.
The classic problems of single-ended dynamic noise reduction systems are (1) the noise level is audibly modulated by the program; and, if the threshold of bandwidth expansion is set high enough to prevent such noise modulation, (2) the program sounds dull because the filter removes more desired high frequency program energy than necessary to eliminate noise modulation. The present invention attacks both these problems in novel ways.
Applicant believes the earliest commercial effort to automatically equalize an audio signal to ensure good quality at all times was the CBS Laboratories "Dynamic Presence Equalizer". This system uses a ratio circuit to determine the ratio between the energy in the midrange frequencies and the total program energy. If the relative energy in the midrange was determined by the ratio circuit to be either excessive or insufficient, the system automatically cut or boosted these frequencies (with an equalizer) to correct the situation.
This system has at least one problem: if the input signal lacked high frequency energy but was nevertheless somewhat noisy, the dynamic presence equalizer would boost the midrange, and with it, the noise. The present invention eliminates this unnatural-sounding artifact. A further problem is that the system requires an analog divider to obtain the ratio. Analog dividers are expensive. The present invention achieves a similar goal with less expensive hardware.
The invention uses a noise reduction system having the general components shown in U.S. Pat. No. 4,696,044. There are, however, significant difference, for example, two of these components are interchanged, changing the characteristics of this prior art system. Specifically, the "log convert" and "absolute value" blocks are interchanged. This changes the dynamic response of the noise reduction system used as part of the present invention. Moreover, as will be seen, the present invention uses a "breakpoint" in controlling bandwidth.